changelog: describe support for static key slot buffers

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/ChangeLog.d/9302.txt b/ChangeLog.d/9302.txt
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+Features
+   * When the new compilation option MBEDTLS_PSA_STATIC_KEY_SLOTS is enabled,
+     key store uses static key slot buffers instead of dynamically allocated
+     ones.
+   * The size of each static key slot buffer is determined by
+     MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE. If not set, its default value
+     is set to accomodate for the largest PSA asymmetric/symmetric key enabled
+     in the build.